This slide shows some rules for the simplified motion of an aircraft.
By simplified motion we mean that some of the four
forces acting on the aircraft are
balanced
by other forces and
that we are looking at only one force and one direction at a time. In
reality, this simplified motion doesn't occur because all of the forces
are interrelated to the aircraft's speed, altitude, orientation, etc.
But looking at the forces ideally and individually does give us some
insight and is much easier to understand.

In an ideal situation, an airplane could sustain a constant speed
and level flight in which the weight would be balanced
by the lift,
and the drag would be balanced by the thrust. The closest example of
this condition is a cruising airliner.
While the weight decreases due to fuel burned, the change is very
small relative to the total aircraft weight. In this situation, the
aircraft will maintain a constant cruise velocity as described by
Newton's first law of motion.

If the forces become unbalanced, the aircraft will move in the
direction of the greater force. We can compute the acceleration which
the aircraft will experience from Newton's second
law of motion

F = m * a

Where a is the acceleration, m is the mass of the
aircraft, and F is the net force acting on the aircraft.
The net force is the difference
between the opposing forces; lift minus weight, or thrust minus drag.
With this information, we can solve for the resulting motion
of the aircraft.

If the weight is decreased while the lift
is held constant, the airplane will rise:

Lift > Weight - Aircraft Rises

If the lift is decreased while the weight is
constant, the plane will fall:

Weight > Lift - Aircraft Falls

Similarly,
increasing the thrust while the drag is
constant will cause the plane to accelerate:

Thrust > Drag - Aircraft Accelerates

And
increasing the drag at a constant thrust
will cause the plane to slow down: